1. Field of the Invention
The present invention provides a media feeding apparatus. More specifically, the present invention provides an auto-compensating mechanism in combination with a step-spring for providing appropriate normal force throughout the feeding of a media stack.
2. Description of the Related Art
Various mechanisms have been utilized to feed media into a printer or other peripheral. Various of these mechanisms utilize a tray or bin in order to support a stack media in which the upper most sheet of the stack may be advanced to a processing station or printing area for printing by a laser printer or inkjet printer, for example. In typical printing or duplicating devices, individual sheets of print media are advanced from the media tray to the processing station by utilizing a paper picking device.
With media picking devices a critical relationship exists between the pick roller and the media stack. More specifically the relationship involves a normal force between the pick roller and the media stack. The normal force must be within an operating range for the pick or media feed process to work properly. When too much normal force exists, multiple sheet of media may be fed resulting in paper jams. When too little normal force exists, media will not feed into the printing area. Some devices utilize a spring loaded paper stack to provide the normal force for picking. Despite extensive tuning of this normal force, usually only a very narrow range of media weights will run reliably on these devices.
Feeding of print media sheets from a stack has been significantly improved by an auto-compensating mechanism (ACM) shown and described in U.S. Pat. No. 5,527,026, issued to Padget et al. which overcomes problems with obtaining proper normal force. Auto-compensating media feeders address prior art issues in media feeding. A pick roll is mounted on the rotating swing arm and rests on the media stack. When the pick roll drive gear is initiated through a gear located on the pivot shaft with the swing arm, a torque is applied to the swing-arm through a gear transmission. The torque rotates the swing arm and pick roll into the media stack. This generates a normal force which is dictated by the buckling resistance of the media being picked. The normal force is no more than is required to buckle a single sheet of media plus the friction resistance between the first and second sheets. When the upper most sheet has moved, the normal force automatically relaxes and, thus, the auto-compensating mechanism will not deliver more normal force than what is required to feed a single sheet of media.
In a C-path feeding system, the ACM is disposed in a generally horizontal position when the media tray contains a full stack of media at upper positions, close to the horizontal, the down force created by the ACM is not high enough to consistently feed the microporous media because the normal force provided by the ACM is low. As the media stack height decreases during operation, the ACM moves through its operating positions during which time the normal force increases. At lower positions, i.e. positions away from the horizontal, the down force is high enough to allow for sheet feeding of the microporous media and the like. These systems are critically affected by various media characteristics including, but not limited to, density, net weight, stiffness and smoothness of the media surface. For example, lightweight media is fairly easy to move from a media stack. However, as media thickness and weight have increased with increased photo printing, the difficulty with consistent feeding throughout a media stack has increased. Even more recently, print feeding difficulties have occurred due to the use of microporous photo paper. The high coefficient of friction between sheets of microporous media tends to remove the ACM from its range of operating torque. Increased down force of the ACM has not alleviated this problem throughout the media stack feeding.
Given the foregoing deficiencies, it will be appreciated that an apparatus is needed which allows consistent media feeding of many types of media.